Recent progress in studies of the South China Sea circulation

The South China Sea (SCS) is a semi-enclosed marginal sea with deep a basin. The SCS is located at low latitudes, where the ocean circulations are driven principally by the Asia-Australia monsoon. Ocean circulation in the SCS is very complex and plays an important role in both the marine environment and climate variability. Due to the monsoon-mountain interactions the seasonal spatial pattern of the sea surface wind stress curl is very specific. These distinct patterns induce different basin-scale circulation and gyre in summer and winter, respectively. The intensified western boundary currents associated with the cyclonic and anticyclonic gyres in the SCS play important roles in the sea surface temperature variability of the basin. The mesoscale eddies in the SCS are rather active and their formation mechanisms have been described in recent studies. The water exchange through the Luzon Strait and other straits could give rise to the relation between the Pacific and the SCS. This paper reviews the research results mentioned above.     

Numerical investigation of the South China Sea deep circulation

Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea（SCS）, the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer（July）, a northeastsouthwest through-flow pattern across the deep basin without a western boundary current in winter（January）,and a transitional pattern in spring and autumn. The sensitivity ...     

Intercomparison of three South China Sea circulation models

1IntroductionTheSouthChinaSeaisthelargesttropicalmarginaldeepsealocatingbetweenthewesternPacificOceanandtheeasternIndianOcean.AsapartofAsia-Australiamaritimecontinent,monsoonisaprimaryfactorforcingtheSouthChinaSeaCurrent(SCSC)variation.Drivenbynortheasterlymonsooninwinterandsouth-westerlymonsooninsummer,respectively,theSCSCbehavesacyclonicgyreandananticy-clonicgyre,correspondingly(Wyrtki,1961;Xuetal.,1982).Owingtotheshortageandexpen-sivenessofdirectobservationsintheSCS,fur-therunder…     

南海中尺度涡年际变化特征及动力机制分析

基于Okubo-Weiss函数方法对20年高度计资料进行涡旋识别,分析了南海中尺度涡的时空分布,初步研究了中尺度涡旋活动的年际变化特征及其可能的动力机制。结果显示,南海中尺度涡旋活动具有较为显著的年际变化特征,通过对涡旋个数、涡区面积、涡动能计算分析表明涡旋活动与ENSO现象遥相关:南海中尺度涡活动在ElNio年较弱,在LaNia年较强。可用风场异常解释南海中尺度涡的年际变化与ENSO现象的负相关关系。ElNio期间南海年平均意义下的东北风场减弱,风应力旋度绝对值减小,从而导致了较弱的涡旋活动,相反LaNia期间强劲的风场导致了涡旋活动增强。     

Anomalous hydrographic and biological conditions in the northern South China Sea during the 1997–1998 El Niño and comparisons with the equatorial Pacific

It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997–1998 El Niño event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997–1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 °C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll-a dropped from 0.2 to 0.1 mg/m³. The monthly area-averaged integrated primary production estimated for the northern SCS area (112–119°E, 15–21°N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997–1998 El Niño event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics.     

南海北部陆缘盆地与坎坡斯盆地

我国南海是西太平洋最大的边缘海,其北部陆缘盆地深水区位于琼东南与珠江口盆地南侧.通过对该地区烃源条件、储层条件、圈闭条件及成藏模式等的初步研究,认为其具有良好的石油地质条件,有效勘探面积和勘探潜力巨大,具有良好的勘探前景.坎坡斯盆地位于巴西大西洋大陆边缘,是一个中生代到古新近纪含油气盆地,其内部的Garoupa, Carapeba, Namorado, Roncador, Marlim等深水油田的地质储量十分丰富,随着各个油田开发的深入,对该地区的深水含油气系统有了更加清晰的了解.本文通过两个地区在构造演化,烃源条件,储层条件,圈闭条件以及成藏模式的对比研究,对南海北部陆缘盆地深水区有了进一步的认识.     

三维斜压模式对冬季南海环流的数值计算

用一个三维、自由表面、原始方程模式对南海环流进行了计算.计算结果表明:黑潮在巴士海峡以西呈一反气旋弯曲流动路径,有一相对高温高盐的水舌从巴士海峡伸入南海,表明有部分黑潮水侵入南海.冬季南海的一些观测事实在模式结果里得到了很好的反映,像冬季逆风向东北方向流动的“南海暖流”和一些中尺度涡旋.同时还分析了巴士海峡沿120.75°E断面的流速和盐度的垂直结构,并同观测结果进行了比较.根据模式结果,我们还进一步讨论了“南海暖流”的形成和驱动机制.     

南海上层环流季节变化的诊断计算

通过一个全球的二维诊断模型,采用Levitus温盐资料和COADS风应力资料,并结合动力计算来研究南海上层环流的季节变化。计算结果与其它模式结果和观测结果非常相似。南海北部(南部)全年存在一气旋式(反气旋式)环流。在冬季气旋式环流几乎占据了整个南海,夏季则以反气旋式环流为主。泰国湾的环流在冬季(夏季)是气旋式的(反气旋的)。南海的西边界流有明显的季节变化,其在冬季从卡里马塔海峡流出南海,夏季部分西边界流从台湾海峡流出南海。越南离岸流在春季就开始出现,其位置比夏季的越南离岸流的位置偏北。     

Circulation in the South China Sea is in a state of forced oscillation: Results from a simple reduced gravity model with a closed boundary

The South China Sea (SCS) is a narrow semi-enclosed basin, ranging from 4°–6°N to 21°–22°N meridionally. It is forced by a strong annual cycle of monsoon-related wind stress. The Coriolis parameter f increases at least three times from the southern basin to the northern basin. As a result, the basin-cross time for the first baroclinic Rossby wave in the southern part of the basin is about 10-times faster than that in the northern part, which plays the most vitally important role in setting the circulation. At the northernmost edge of SCS, the first baroclinic Rossby wave takes slightly less than 1 year to move across the basin, however, it takes only 1–2 months in the southernmost part. Therefore, circulation properties for a station in the model ocean are not solely determined by the forcing at that time instance only; instead, they depend on the information over the past months. The combination of a strong annual cycle of wind forcing and large difference of basin-cross time for the first baroclinic Rossby wave leads to a strong seasonal cycle of the circulation in the SCS, hence, the circulation is dominated by the forced oscillations, rather than the quasi-steady state discussed in many textbooks.The circulation in the SCS is explored in detail by using a simple reduced gravity model forced by seasonally varying zonal wind stress. In particular, for a given time snap the western boundary current in the SCS cannot play the role of balancing mass transport across each latitude nor balancing mechanical energy and vorticity in the whole basin. In a departure from the steady wind-driven circulation discussed in many existing textbooks, the circulation in the SCS is characterized by the imbalance of mechanical energy and vorticity for the whole basin at any part of the seasonal cycle. In particular, the western boundary current in the SCS cannot balance the mass, mechanical energy, and vorticity in the seasonal cycle of the basin. Consequently, the circulation near the western boundary cannot be interpreted in terms of the wind stress and thermohaline forcing at the same time. Instead, circulation properties near the western boundary should be interpreted in terms of the contributions due to the delayed wind stress and the eastern boundary layer thickness. In fact, there is a clear annual cycle of net imbalance of mechanical energy and vorticity source/sink. Results from such a simple model may have important implications for our understanding of the complicated phenomena in the SCS, either from in-situ observations or numerical simulations.     

南海东部中尺度涡生成机制

利用高度计海面高度异常数据和非线性1½层约化重力模式研究了南海东部中尺度涡的生成机制。模式结果表明，南海内区风场是南海东部中尺度涡生成的主要驱动力，且南海内区高频风场能解释约54%的南海东部中尺度涡。从西太平洋传来的信号同样有十分重要的作用，由西太区域高频风场大致能解释南海东部40%的中尺度涡。风驱动的赤道附近的海面异常信号能经过锡布图通道和民都洛海峡传播到吕宋岛西海岸，其中有部分能量会以罗斯贝波的形式往西传播。这种信号在西传的过程中会发生不稳定，可能形成孤立的涡旋。     

南海上层环流观测研究进展

回顾了近50a来南海环流研究的进展，重点介绍了近期有关南海上层总环流的观测研究成果，并就南海季风急流、南海暖流、南海南部的次海盆尺度环流，以及南海东北部环流的几个问题进行了专门讨论。     

Study on circulation and meso-scale eddies in the South China Sea in summer of 1998

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Wind relaxation as possible cause of the South China Sea Warm Current

Winter appearance of a northeastward warm current off the southern coast of China against gale force winds is well documented but lacks a plausible explanation. Relaxation of northeasterly winds is envisaged here as a possible cause of the South China Sea Warm Current in winter. A three-dimensional circulation model for the South China Sea is first driven to equilibrium by climatological forcings. Thereafter, wind forcing is relaxed from the 15th day of each month for 9 days. In winterlike months from December to April, the wind relaxation invariably triggers a northeastward current of which the location and alongshore span are comparable to that of the observed warm current. This current is driven by the pressure gradient along the northwestern boundary of the South China Sea, sea level being high to the southwest and low to the northeast. The sea level gradient is built up by the monsoon-driven southwestward coastal current along the northwestern boundary and, after wind relaxes, triggers a return current and a sea level drop that expand southwestward from the southern coast of China to the east coast of Vietnam. The current is initially barotropic, becoming increasingly baroclinic in time as warm waters from the south are advected northeastward. The model also suggests that the sea level gradient is present in most of the months of the year, but is not as dramatic as in winter to trigger fundamental changes in the circulation of the South China Sea.     

A 3-dimensional baroclinic circulation model of the South China Sea

1 INTRODUCTIONThe SoUth China Sea (SCS) is the largest coastal sea of China. It is of great interestto researchers because of its abundant resources and geographical location. The SCS is inthe tropicaI and sub-tropical monsoon wind zone of the north-western Pacific and isSurrounded by China, Vietnam, Malaysia, Indonesia and the Philippines. lt has a complicatedRecclved 2 l March 200ltoPograPhy and is cormected with the Peeific through thc Bashi Channel which has a dePthof over 3…     

Impacts of a wind stress and a buoyancy flux on the seasonal variation of mixing layer depth in the South China Sea

The seasonal variation of mixing layer depth(MLD) in the ocean is determined by a wind stress and a buoyance flux.A South China Sea(SCS) ocean data assimilation system is used to analyze the seasonal cycle of its MLD.It is found that the variability of MLD in the SCS is shallow in summer and deep in winter,as is the case in general.Owing to local atmosphere forcing and ocean dynamics,the seasonal variability shows a regional characteristic in the SCS.In the northern SCS,the MLD is shallow in summer and deep in winter,affected coherently by the wind stress and the buoyance flux.The variation of MLD in the west is close to that in the central SCS,influenced by the advection of strong western boundary currents.The eastern SCS presents an annual cycle,which is deep in summer and shallow in winter,primarily impacted by a heat flux on the air-sea interface.So regional characteristic needs to be cared in the analysis about the MLD of SCS.     

Upper Ocean Sensitivity to Wind Forcing in the South China Sea

The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) has been used to investigate the sensitivity of the upper South China Sea (SCS) circulation to various atmospheric wind forcing products. A 1/16° 6-layer, thermodynamic Pacific Ocean north of 20°S version of NLOM has been integrated using observed climatological monthly mean winds (Hellerman and Rosenstein, 1983) and climatologies based on two atmospheric prediction models: the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP). ECMWF products include the 10 meter winds (at both 1.125° and 2.5° resolution) and surface stresses (1.125°). The NCEP forcing (1.875°) is a surface stress product. Significant differences exist in the wind stress curl patterns and this is reflected in the upper ocean model response, which is compared to observational data. The model experiments suggest the generation of the West Luzon Eddy is controlled by positive wind stress curl. The degree of Kuroshio intrusion into the SCS, however, is not affected by wind stress curl but is governed by the coastline geometry of the island chain within Luzon Strait. The summertime offshore flow from the Vietnamese coast is present in all simulations but the dipole structure on either side of the jet is variable, even among experiments with similar wind stress curl patterns. The ECMWF surface stresses exhibit spurious coastal wind stress curl patterns, especially in locations with significant orographic features. This manifests itself in unrealistic small scale coastal gyres in NLOM. High resolution basin-scale and coastal models might be adversely affected by these stresses. This revised version was published online in July 2006 with corrections to the Cover Date.     

南海TOPEX海面高度资料的混合同化试验

应用普林斯顿海洋模式（POM），对冬季南海由TOPEX／Poseidon得到的卫星遥感海面高度资料进行混合（blending)同化试验。混合同化的权重系数由以下两者决定：南海POM模式对冬季风强迫产生响应的海面高度场之标准方差；对应期间ＴＯＰＥＸ海面高度资料的标准方差。同化结果表明，混合同化方法不失为一种简单而又有效的同化方案。同化得到的南海环流结构与未同化的模式响应场比较可以发现：海面高度资料的同化试验能够有效地修正南海环流的若干大尺度特征，特别是对冬季黑潮入侵南海东北部的动力过程，同化结果有了更准确的描述。同时，另一个重要的修正表现在：同化试验中瞬变的中等尺度涡旋得到加强，体现了南海海洋天气尺度涡被资料同化所“唤醒”（trigger)。这种“唤醒”不仅明显地反映在表层环流场中，对南海次表层动力、热盐结构也有一定的影响。     

南海热流分布特征

南海地热异常明显与主要构造断裂带和水热/岩浆活动有关。东部平行于马尼拉海沟的一条SN向低热流异常带起因于南海洋壳对吕宋岛的俯冲。南沙海槽及其南部陆缘的地温场比较复杂。南部的曾母盆地是一个显著的高地热异常区,它起因于年轻的构造拉张,其地幔热流高达中央海盆洋壳的地幔热流值。西南次海盆也是一个高地热异常区,虽然该次海盆形成较早,但与年轻的构造拉张有关。热流资料的分析结果表明,南海中央海盆西缘断裂带、西南次海盆和曾母盆地构成的NE向高热流异常带可能是一个大型的现代构造拉张带。     

South China Sea wave characteristics during typhoon Muifa passage in winter 2004

The responses to tropical cyclones of ocean wave characteristics in deep water of the western Atlantic Ocean have been investigated extensively, but not the regional seas in the western Pacific such as the South China Sea (SCS), due to a lack of observational and modeling studies there. Since monsoon winds prevail in the SCS but not in the western Atlantic Ocean, the SCS is unique for investigating wave characteristics during a typhoon’s passage in conjunction with steady monsoon wind forcing. To do so, the Wavewatch-III (WW3) is used to study the response of the SCS to Typhoon Muifa (2004), which passed over not only deep water but also the shallow shelf of the SCS. The WW3 model is forced by the NASA QuikSCAT winds and tropical cyclone wind profile model during Typhoon Muifa’s passage from 0000UTC 16 on November to 1200UTC on 25 November 2004. The results reveal the unique features of the SCS wave characteristics in response to Muifa, such as non-decaying, monsoon-generated swell throughout the typhoon period and strong topographic effects on the directional wave spectrum.     

冬季南海北部中尺度涡旋的数值研究

南海环流的一个主要特征是上层海洋环流具有多涡结构,海洋中尺度涡旋的演变(时间上的生消和空间上的迁移)是南海环流季节调整的可能方式。文中依据卫星遥感海面高度资料和实际海洋观测所揭示的南海北部存在中尺度涡旋体系的基本事实,采用一个改进了涡分辨(eddy-resolving)普林斯顿海洋模式(POM),对冬季处于强盛的东北季风强迫以及黑潮在巴士海峡入侵的共同作用下的南海北部环流的中尺度涡旋体系进行了数值研究,初步再现了冬季南海北部中尺度涡的生命史。计算结果表明,在实际的气候冬季风应力驱动下,具有的实际侧边界地形的南海北部呈现有强烈的中尺度涡旋。文中探讨了中尺度涡的垂直结构、温盐场的配置以及大尺度水平辐合辐散、海洋垂直运动与之相关的时空结构。由此可以得知,在冬季南海北部中尺度涡旋生命史的不同阶段,上述动力学因子的重要性是相对的。不同的敏感性试验表明,斜压调整是形成冬季南海中尺度涡旋体系的决定性因子;边界的入流和风应力驱动是影响中尺度涡旋运动的主要因素。